1. Field of the Invention
The present invention relates to an oscillator, and more particularly, to a low-phase noise oscillator with a microstrip resonator.
2. Description of the Prior Art
Although nearly all the current local area networks (LAN) were constructed using a wired network structure, wireless network structures are becoming increasingly popular, especially at university campuses, hospitals, and manufacturing plants. A wireless network structure is ideal for a small area communication system because of its low-cost and simple construction. To guarantee that wireless network equipment is compatible, the institute of electrical and electronics engineers (IEEE) set a first standard IEEE 802.11 in 1997, and then made amendments IEEE 802.11a, IEEE 802.11b in September 1999. The standards define elementary data transmission methods and service transmission frequency. These standards are applied in industrial, scientific, and medical (ISM) frequency bands, including 902xcx9c928 MHz, 2.4xcx9c2.4835 GHz, 5.150xcx9c5.350 GHz, and 5.725xcx9c5.850 GHz.
Superheterodyne, heterodyne, and zero IF or direct conversion structure, as used in radio before, are currently still the most popular communication structures. Because a baseband circuit is mainly used to process signals, such as modulating, demodulating, encoding or decoding signals, it is usually manufactured using an integrated circuit manufacturing process. However, IF circuits and RF circuits are mainly used to receive and transmit radio signals, so they must bear a high signal-to-noise ratio (SNR). Therefore, each component, particularly an oscillator, in a wireless communication circuit must bear a high Q factor.
A resonator is an important element in many electrical systems and can be used to fabricate a filter or an oscillator. In manufacturing a resonator, many factors should be taken into consideration, such as size, cost, Q factor, and dependability. Some popular resonators are illustrated as follows. The first one is a resonator formed by a plurality of capacitors and inductors. It has a low Q factor because of the power consumption of the capacitors and the inductors. The second one is a coaxial resonator, which is large and expensive. The third one is a cavity resonator. It has a rectangular, cylindrical or spherical shape and is formed of conductive materials, and consequently has the characteristics of low power consumption and high Q factor, but large volume. The fourth one is a dielectric resonator, which is formed by a dielectric sphere. Although the dielectric resonator has a small volume, low power consumption, and high Q factor, manufacturing a dielectric resonator is still expensive. The last resonator is a microstrip resonator, which is formed by disposing a conductive strip onto a circuit board. Because one terminal of a microstrip resonator is open, microwave radiation will consume power so as to make the microstrip resonator have a low Q factor.
Standard integrated circuits are planar circuits, so only those resonators having a planar structure, such as the microstrip resonator, are suitable for designing a microwave integrated circuit (MMIC) or radio frequency integrated circuit (RFIC). A conventional microstrip resonator has a low Q factor and consequently designing a low-phase-noise oscillator with such microstrip resonator is difficult.
It is therefore a primary objective of the claimed invention to provide an oscillator with a high Q factor.
According to the claimed invention, a low-phase-noise oscillator with a microstrip resonator for generating a target signal with a predetermined frequency is provided. The oscillator includes an oscillating circuit for generating a plurality of signals with different frequencies, and a first resonator. The first resonator includes an input port connected to an output port of the oscillating circuit, an output port for outputting the target signal, a circuit board having a metal membrane which functions as a ground layer for providing a reference voltage, a first microstrip line with a first predetermined length being positioned on the circuit board and coupling with the metal membrane for forming a first transmission line structure. The first microstrip line has a first terminal and a second terminal. The first terminal is connected to the input port of the first resonator. The predetermined frequency of the target signal is determined according to the first predetermined length. The first resonator also includes a second microstrip line with a second predetermined length being positioned on the circuit board and coupling with the metal membrane of the circuit board for forming a second transmission line structure. The second microstrip line has a first terminal and a second terminal. The first and the second microstrip lines are conductive bars and the first microstrip line is parallel with the second microstrip line without any contacts. Either the second terminal of the first microstrip line or the second terminal of the second microstrip line is connected to the output port of the first resonator. When the oscillating circuit generates signals with different frequencies to the input port of the first resonator, a signal with the predetermined frequency is outputted from the output port of the first resonator to be the target signal through an electromagnetic coupling between the first and second microstrip lines of the first resonator.
It is an advantage of the claimed invention that an oscillator with a microstrip resonator can provide a high Q factor and consequently a low phase noise.
These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.